1. Field of the Invention
This invention relates to a method of manufacturing a shock absorbing type steering shaft incorporated in the steering apparatus of an automobile and utilized to transmit the movement of a steering wheel to a steering gear.
2. Related Background Art
In a steering apparatus for an automobile, a mechanism as shown in FIG. 9 of the accompanying drawings is used to transmit the movement of a steering wheel to a steering gear. In FIG. 9, the reference numeral 1 designates a first steering shaft having a steering wheel 2 fixed to the upper end portion thereof, and the reference numeral 3 denotes a steering column. This steering column 3 is fixed to the lower surface of an instrument panel 6 by upper and lower brackets 4 and 5. The first steering shaft 1 is rotatably inserted in the steering column 3.
The upper end portion of a second steering shaft 8 is connected through a first universal joint 7 to the lower end portion of the first steering shaft 1 which protrudes from the lower end opening of the steering column 3. Further, the lower end portion of the second steering shaft 8 is connected through a second universal Joint 9 to a third steering shaft 10 leading to a steering gear (not shown).
With such a construction, the movement of the steering wheel 2 is transmitted to the steering gear through the first steering shaft 1 inserted in the steering column 3, the first universal joint 7, the second steering shaft 8, the second universal joint 9 and a third steering shaft 10 to give a steering angle to wheels.
In the steering mechanism thus constructed, the steering column 3 and the steering shafts 1, 8, 10 are usually made into a shock absorbing type in which the full length shortens due to a shock in order to protect a driver during collision. As such a shock absorbing type steering shaft, a structure in which an outer shaft and an inner shaft serration-engaged with each other are coupled together by synthetic resin is described, for example, in Japanese Patent Application Laid-Open No. 2-286468. Also, in Japanese Utility Model Application Laid-Open No. 1-58373, there is described a structure in which male serration grooves are formed at two locations on the outer peripheral surface of an inner shaft and a female serration groove is formed on the inner peripheral surface of an outer shaft and these male and female grooves are pressure-fitted to each other.
However, in the case of the structure described in the above-mentioned Japanese Patent Application Laid-Open No. 2-286468, the coupling of the outer shaft and the inner shaft is done by only synthetic resin and therefore, it is conceivable that sufficient torsional durability is not obtained due to a deficiency in heat resistance under certain conditions, such as when it is installed within an engine compartment and liable to become hot. Also, in the case of the structure described in the above-mentioned Japanese Utility Model Application Laid-Open No. 1-58373, the work of making the phases of the male and female serration grooves coincident with each other to pressure-fit these grooves to each other becomes cumbersome and the cost of manufacture increases.
As a structure which can eliminate such inconveniences, a shock absorbing type steering shaft as shown in FIGS. 10 to 16 of the accompanying drawings is described in Japanese Utility Model Application Laid-Open No. 6-8150. This shock absorbing type steering shaft 11 is constructed such that an outer shaft 12 and an inner shaft 13 are combined for relative displacement in an axial direction (the left to right direction as viewed in FIG. 10), whereby the full length shortens when an impact force in the axial direction is applied.
The outer shaft 12 as a whole is of a tubular shape and one end portion (the left end portion as viewed in FIGS. 10 and 14) thereof is subjected to drawing, whereby a small-diametered portion 14 is formed in this end portion. A female serration 15 is formed on the inner peripheral surface of this small-diametered portion 14. The inner shaft 13 as a whole is of a tubular shape and one end portion (the right end portion as viewed in FIGS. 10 and 11) thereof is widened to thereby form a large-diametered portion 16. A male serration 17 is formed on the outer peripheral surface of the large-diametered portion 16 and engaged with the female serration 15.
Also, the fore end portion (the right end portion as viewed in FIGS. 10 and 11) of the large-diametered portion 16 is squeezed a little in the diametral direction thereof, whereby a first deformed portion 18 of an elliptical cross-sectional shape is formed over a length L. The major axis d.sub.1 of this first deformed portion 18 is larger than the diameter d.sub.0 of the body portion of the large-diametered portion 16, and the minor axis d.sub.2 of the first deformed portion 18 is smaller than the diameter d.sub.0 (d.sub.1 &gt;d.sub.0 .gtoreq.d.sub.2). The diameters of the large-diametered portion 16 on which the male serration 17 is formed are all represented by the diameter (pcd) of that portion of the serration which corresponds to a pitch circle.
The fore end portion (the left end portion as viewed in FIGS. 10 and 14) of the small-diametered portion 14 is also squeezed a little in the diametral direction thereof, whereby a second deformed portion 19 of an elliptical cross-sectional shape is formed over a length L. The major axis D.sub.1 of this second deformed portion 19 is larger than the diameter D.sub.0 of the body portion of the small-diametered portion 14, and the minor axis D.sub.2 of the second deformed portion 19 is smaller than the diameter D.sub.0 (D.sub.1 .gtoreq.D.sub.0 &gt;D.sub.2). The diameters of the small-diametered portion 14 on which the female serration 15 is formed are all represented by the diameter (pcd) of that portion of the serration which corresponds to a pitch circle.
Also, the diameter D.sub.0 of the small-diametered portion 14 is made slightly larger than the diameter d.sub.0 of the large-diametered portion 16 (D.sub.0 &gt;d.sub.0) so that the female serration 15 and the male serration 17 may be loosely engaged with each other in portions other than the first and second deformed portions 18 and 19. However, the major axis d.sub.1 of the first deformed portion 18 is a little larger than the diameter D.sub.0 of the body portion of the small-diametered portion 14 (d.sub.1 &gt;D.sub.0) and the minor axis D.sub.2 of the second deformed portion 19 is a little smaller than the diameter d.sub.0 of the body portion of the large-diametered portion 16 (D.sub.2 &lt;d.sub.0).
The outer shaft 12 and inner shaft 13 of the shapes as described above are combined together as shown in FIG. 10 to thereby provide the shock absorbing type steering shaft 11. That is, the large-diametered portion 16 formed on one end portion of the inner shaft 13 is located inside the small-diametered portion 14 formed on one end portion of the outer shaft 12, and the female serration 15 on the inner peripheral surface of the small-diametered portion 14 and the male serration 17 on the outer peripheral surface of the large-diametered portion 16 are brought into engagement with each other. In this state, the first deformed portion 18 formed on the fore end portion of the large-diametered portion 16 is pushed into the base end portion (the right end portion as viewed in FIGS. 10 and 14) of the small-diametered portion 14 while being elastically deformed (or plastically deformed). The second deformed portion 19 formed on the fore end portion of the small-diametered portion 14 is pushed into the base end portion (the left end portion as viewed in FIGS. 10 and 11) of the large-diametered portion 16 while being also elastically deformed (or plastically deformed).
Accordingly, in the state in which the outer shaft 12 and the inner shaft 13 are combined together as shown in FIG. 10, the outer peripheral surface of the first deformed portion 18 is frictionally engaged with the inner peripheral surface of the base end portion of the small-diametered portion 14 and the inner peripheral surface of the second deformed portion 19 is frictionally engaged with the outer peripheral surface of the base end portion of the large-diametered portion 16. As a result, the outer shaft 12 and the inner shaft 13 are coupled together for the transmission of a rotational force between the two shafts 12 and 13 but against relative displacement in the axial direction as long as a strong force is not applied.
The coupling of the outer shaft 12 and the inner shaft 13 is thus effected by the pressure fitting of the first and second deformed portions 18 and 19 formed on the metallic outer shaft 12 and inner shaft 13 to the partner members and therefore, the heat resisting property of the coupling portion becomes sufficient and it never happens that the supporting force of the coupling portion becomes deficient depending on use conditions. Also, the first and second deformed portions 18 and 19 are provided at two axially spaced apart locations in the coupling portion between the outer shaft 12 and the inner shaft 13 and therefore, the bending rigidity of the coupling portion between the outer shaft 12 and the inner shaft 13 is also sufficiently secured.
Further, when a strong force is applied in the axial direction during collision, the outer shaft 12 and the inner shaft 13 are displaced relative to each other in the axial direction against a frictional force exerted on the pressure-fitted portions by the first and second deformed portions 18 and 19 to thereby shorten the full length of the shock absorbing type steering shaft 11. In the case of such a shock absorbing type steering shaft 11, the force required to shorten the full length suffices if it overcomes the frictional force exerted on the above-described two pressure-fitted portions. Accordingly, a collapse load required to shorten the full length of the shock absorbing type steering shaft 11 is stable without becoming great, thereby effectively preventing a great impact force from being applied to a driver's body which has collided against the steering wheel in case of a collision accident.
The collapse load can be arbitrarily adjusted by changing the length L and/or the major axis d.sub.1 and the minor axis D.sub.2 of the first and second deformed portions 18 and 19. Also, the two pressure-fitted portions are provided on the opposite end portions of the portion of engagement between the small-diametered portion 14 formed on one end portion of the outer shaft 12 and the large-diametered portion 16 formed on one end portion of the inner shaft 13 and therefore, the magnitude of the force required to shorten the full length of the shock absorbing type steering shaft 11 becomes small from a predetermined point (after the amount of contraction of the shock absorbing type steering shaft 11 exceeds the length L and the first deformed portion 18 comes off the small-diametered portion 14 and the second deformed portion 19 comes off the large-diametered portion 16) as shown in FIG. 17 of the accompanying drawings. The stroke amount (the amount of contraction) required until the force thus becomes small can be arbitrarily set by changing the length L of the first and second deformed portions 18 and 19.
To form the second deformed portion 19 on one end portion of the outer shaft 12 constituting such a shock absorbing type steering shaft 11, the outer shaft 12 and the inner shaft 13 are combined together into a state as shown in FIG. 18 of the accompanying drawings wherein the fore end portion (the left end portion as viewed in FIG. 18) of the small-diametered portion 14 is protruded a little from the base end portion (the left end portion as viewed in FIG. 18) of the large-diametered portion 16, and in this state, the fore end portion of the small-diametered portion 14 is squeezed in the diametral direction thereof to thereby form the second deformed portion 19. Also, to form the first deformed portion 18 on one end portion of the inner shaft 13, a mold 26 having an elliptical cross-section as shown in FIG. 19 of the accompanying drawings is pushed into the fore end portion of the large-diametered portion 16 formed on one end of the inner shaft 13 to thereby plastically deform this fore end portion.
The shock absorbing type steering shaft 11 constructed and acting as described above can have a sufficient heat resisting property and rigidity and yet can collapse under a sufficiently low load and can thus effectively improve the safety of the driver during a collision accident, but it is desired to simplify the manufacturing work to thereby reduce the manufacturing cost. Heretofore, the work of forming the second deformed portion 19 on the outer shaft 12 and the work of forming the first deformed portion 18 on the inner shaft 13 have been done discretely. Therefore, the following two problems (1) and (2) have arisen.
(1) Since these deformed portions 19 and 18 are formed discretely, two steps become necessary to form the deformed portions 19 and 18.
(2) Also, since the deformed portions 19 and 18 are formed discretely, the adjustment of the fitting strength of the outer shaft 12 and the inner shaft 13 by these deformed portions 19 and 18 becomes cumbersome.
Due to these causes (1) and (2), the manufacturing cost of the shock absorbing type steering shaft 11 increases. The shock absorbing type steering shaft of the present invention has been made in order to reduce the manufacturing cost in view of such circumstances.